This invention relates to electrodes for use in electrolytic cells and to a method for preparing the electrodes. More specifically, this invention relates to fluid-permeable electrodes suitable for use as anodes and cathodes in electrolytic hydrogen generation cells in which it is necessary to continuously remove the products of the electrochemical reaction and to a method for preparing the electrodes.
An assured long-term supply of inexpensive energy is essential for the maintenance and growth of a modern industrial society. The growing demand for energy, combined with the realization that fossil fuel sources are limited, has made it critical to examine alternate sources of energy and to develop methods for extending the lifetimes of our remaining fuels.
As the supply of natural gas and oil decreases, increased emphasis is being placed on methods of obtaining liquid and gaseous fuels from coal. Nearly all coal conversion methods involve increasing the hydrogen-to-carbon ratio of the hydrocarbons in the product. This is accomplished by either adding hydrogen to or removing carbon from the coal. The lighter the final hydrocarbon, the more the hydrogen-to-carbon ratio must be increased.
When external hydrogen is supplied to a coal conversion process, quantitatively less carbon is rejected in the process of adjusting the hydrogen-to-carbon ratio. However, depending upon the process, hydrogen production itself can consume large quantities of hydrocarbons. Thus, sources of hydrogen other than from fossil fuels are necessary to efficiently utilize our alternate reserves of fossil fuels. The production of hydrogen from water becomes important, particularly if the process can be energized by nuclear heat, solar energy or high-temperature waste heat from other industrial processes.
The theoretical voltage required to decompose water is 1.23 V, although many commercial electrolytic processes require over 2.0 V to accomplish this. Thus many processes for splitting water are presently under development which will require only an input of thermal energy or thermal energy coupled with a small quantity of electrical power in order to keep costs and energy consumption at a low level. One example of the latter is a two-step hybrid electrochemical/thermochemical process cycle called the sulfur cycle water decomposition system. This process in its most general form consists of two chemical reactions--one for producing oxygen and the other for producing hydrogen. The production of oxygen occurs via the thermal reduction of sulfur trioxide obtained from sulfuric acid. EQU H.sub.2 SO.sub.4 .fwdarw.H.sub.2 O+SO.sub.3 .fwdarw.H.sub.2 O+SO.sub.2 +1/2 O.sub.2
The process is completed by using the sulfur dioxide from the thermal reduction step to depolarize the anode of a water electrolyzer. The overall reduction occurring electrochemically is EQU 2H.sub.2 O.fwdarw.H.sub.2 +H.sub.2 O
This is comprised of the reactions: EQU Cathode 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 EQU Anode H.sub.2 SO.sub.3 +H.sub.2 O.fwdarw.2H.sup.+ +H.sub.2 SO.sub.4 +2e.sup.-
The net result is the decomposition of water into hydrogen and oxygen while sulfur oxides are involved as recycling intermediates. Although electrical power is required in the electrolyzer, the quantities are much smaller than those necessary in conventional electrolysis, only 0.17 volts being needed at unit activity for reactants and products, which is less than 15% of those required in conventional electrolysis.
However, early tests showed that the current could not be maintained for any period of time and that at the cell anode where sulfurous acid is oxidized to sulfuric acid the anode potential quickly went to oxygen evolution potential. This was found to be due to the low solubility of SO.sub.2 and the fact that the sulfurous acid can only reach the anode by diffusion and that once the material is oxidized there is no tendency to leave the vicinity of the anode surface. A number of solutions have been tried to increase contact between the SO.sub.2 and the anode. For example, porous and hollow electrodes have been tried, as has circulating the anolyte around flooded platinized carbon or platinized platinum electrodes, but cell voltages continued to be too high.